In recent years, wireless data communication in domestic and enterprise environments have become increasingly commonplace and an increasing number of wireless communication systems have been designed and deployed. In particular, the use of wireless networking has become prevalent and wireless network standards such as IEEE 801.11a and IEEE 801.11 g have become commonplace.
The requirement for increasing data rates, communication capacity and quality of services has led to continued research and new techniques and standards being developed for wireless networking. One such standard is the IEEE 801.11n standard which is currently under development. IEEE 801.11n is expected to operate in the 2.4 GHz or 5 GHz frequency spectrum and promises data rates of around 100 Mbps and above on top of the MAC layer. IEEE 801.11n will use many techniques which are similar to the earlier developed IEEE 801.11a and IEEE 801.11 g standards. The standard is to a large extent compatible with many of the characteristics of the earlier standards thereby allowing reuse of techniques and circuitry developed for these. For example, as in the previous standards IEEE 801.11a and IEEE 801.11 g, IEEE 801.11n will use Orthogonal Frequency Division Multiplex (OFDM) modulation for transmission over the air interface.
The frame or packet formats employed by the IEEE 801.11 a/g/n standards can be understood with reference to the open system interconnection (OSI) model, which defines the application, presentation, session, transport, network, data link, and physical layers. The data link layer includes a logical link control (LLC) layer and a media access control layer. The MAC layer controls how to gain access to the network, and the LLC layer controls frame synchronization, flow control and error checking. The physical layer (PHY) transmits signals over the network. FIG. 1 shows the LLC, MAC and PHY layers along with the IEEE 801.11 a/g/n frames with which they are associated. As shown, each MAC service data unit (MSDU) or frame 11, received from a logic link control layer (LLC) 10, is appended with a MAC header and a frame check sequence (FCS) trailer, at the MAC layer 20, to form a MAC layer protocol data unit (MPDU) or frame 21. At the physical layer, the MPDU is received as a physical layer service data unit (PSDU) or frame 22. At the physical layer 30, a physical layer convergence procedure (PLCP) header, a PLCP preamble, and tail and pad bits are attached to the PSDU frame 22 to form a physical layer protocol data unit (PPDU) or frame 31 for transmission on the channel.
In order to improve efficiency and to achieve the high data rates, IEEE 801.11n is planned to introduce a number of advanced techniques. For example, IEEE 801.11n communication is expected to be typically based on a plurality of transmit and receive antennas. Furthermore, rather than merely providing diversity from spatially separated transmit antennas, IEEE 801.11n will utilise transmitters having at least partially separate transmit circuitry for each antenna thus allowing different sub-signals to be transmitted from each of the antennas. The receivers may receive signals from a plurality of receive antennas and may perform a joint detection taking into account the number and individual characteristics associated with each of the plurality of transmit antennas and receive antennas. Specifically, IEEE 801.11n has seen the introduction of a Multiple-Transmit-Multiple-Receive (MTMR) antenna concept which exploits Multiple-Input-Multiple-Output (MIMO) channel properties to improve performance and throughput. MIMO processing operates in conjunction with information located in PPDU frame or packet.
One class of MTMR techniques that is specified in IEEE802.11n is spatial mapping. Spatial mapping techniques include direct mapping, cyclic shift diversity, beamforming and spatial expansion techniques. In spatial expansion, space expanded symbols are transmitted from spatially separate antennas. The spatial expansion provides separate streams for each of the spatially separate antennas. More specifically, the spatial expansion or coding includes encoding a stream of symbols to provide separate encoded streams for separate antennas. Each encoded stream is distinct. For example, differential delays can be imposed upon different streams by imposing different phase rotations on the samples of each of the streams.
Spatial mapping techniques can be used to provide range extension i.e., to achieve a higher signal to noise ratio at the receiver, thus allowing data to be to be properly decoded at a larger distance from the transmitter. One way to provide range extension uses open loop processing, in which the transmitter does not have any knowledge concerning the state of the channel over which the signal is transmitted. Spatial expansion is one spatial mapping technique that employs open loop processing. Alternatively, closed loop processing may be employed, in which the receiver provides the transmitter with channel state information that can be used to further increase the signal to noise ratio of the received signal. One example of a spatial mapping technique that employs closed loop processing is beamforming.
One problem that arises when spatial mapping techniques are applied to IEEE 801.11n PPDU frames is that long range spatial expansion techniques cannot be applied to sounding PPDUs, thus limiting the range of the future transmissions using beamforming. This problem arises because sounding PPDUs do not fulfil the requirements of the IEEE 801.11n standard, which specifies that the spatial mapping matrix should be formed of orthogonal columns with the same norm.